THE DYNAMICS OF PARTIALLY MOLTEN CRUST IN DOMES, CORE COMPLEXES, AND OROGENIC PLATEAUX

The internal structure of migmatite terrains records the dynamics of large-magnitude horizontal and vertical flow of low-viscosity (partially molten) crust. The low-viscosity crust is the product of thermal relaxation and/or heating, depending on the larger geodynamic setting in which the orogen evolves (continental subduction, crustal thickening, lithosphere foundering, slab break-off, slab window, etc.). During orogenic collapse, upward flow of low-viscosity crust is controlled by normal to oblique extension of the upper crust, leading to the emplacement of migmatite-cored metamorphic core complexes. Upward flow results in decompression melting that lowers viscosity, and in lateral pressure gradients that attract adjacent low viscosity crust toward the zone of extension. The relative importance of crustal thickness and geothermal gradient, which determine the thickness of partially molten crust, is investigated by thermomechanical numerical modeling for various rates of extension. Results show that upper-crust extension is dynamically linked to lower crustal flow until much of the lower crust material is drained or until boundary conditions change. Therefore, a typical crustal section of a collapsing orogen involves partial convection of material where extension of upper crust (divergent motion) is underlain by lateral flow (convergent motion) and exhumation (upward flow) of the deep crust.

This general concept explains simultaneous lower crust contraction (upright folds, vertical shear zones) and upper crust extension/basin development in and around metamorphic core complexes and migmatite domes. Examples from Cordilleran core complexes (North America), Montagne Noire (French Massif Central), Fosdick Mts (Antarctica), and Naxos (Greece/Aegean Sea) illustrate this general process. Implications include better understanding of (1) heat and mass transfer in orogens; (2) the role of oblique divergence in the development of migmatite domes, including the dynamics of melt migration and development of anatectic granite sheets; and (3) contraction and extension in space and time at various levels of orogens.